v0.20.2: Feature Update

As the releases before, this release has 100% test coverage.

Deprecration WARNING

Support for Go 1.10, 1.11 and 1.12 will be dropped in a future release.
The errors.Is and errors.As features greatly simplify error testing and handling and will likely be introduced in the near future.

Improve autocompletion behaviour.

• Pass autocompletion entries to children.

From v0.20.0 all options started being passed to children commands.
Their completion entries were missing.

• Separate internal option completion between flags that don't expect an argument and options that do.
  When an option that expects an argument is found, the given argument won't break the completion chain.
  Only one argument is supported per option.

• Don't break autocompletion chain when there is an option in the chain that accepts an argument with =.
  For example: program --profile=dev <tab><tab> will show completions for program.